Real life pop-rock group The Shadows, Cliff Richard's regular backing band, feature in numerous guises throughout the film: including as French cyclists; musicians "Les Shadows"; and members of the Greek Presidential Guard.
Three London buses were used in the filming of Summer Holiday, posing as bus number RT1881 and fitted with the fake number plate WLB 991. The buses - 1949 London Transport AEC Regent III RT models - were in fact RT2305 (original number plate KGU 334); RT2366 (KGU 395); and RT4326 (NLE 990). A real bus had previously been coded RT1881 by London Transport, with the number plate LLU 767.
Source: Buses On Screen